Bug 1264830 - Part 1: Add an nsStyleAutoArray array type, similar to AutoTArray<...,1> but memmovable. r=bholley

Existing uses of AutoTArray in style structs makes them non-memmovable.
We introduce this AutoTArray-alike class for use by those style struct
members that really do need to use an auto array's built-in allocation.

layout/style/nsStyleStruct.h

@@ -453,6 +453,65 @@ struct nsStyleColor
   nscolor mColor;                 // [inherited]
 };
 
+/**
+ * An array of objects, similar to AutoTArray<T,1> but which is memmovable. It
+ * always has length >= 1.
+ */
+template<typename T>
+class nsStyleAutoArray
+{
+public:
+  // This constructor places a single element in mFirstElement.
+  enum WithSingleInitialElement { WITH_SINGLE_INITIAL_ELEMENT };
+  explicit nsStyleAutoArray(WithSingleInitialElement) {}
+  nsStyleAutoArray(const nsStyleAutoArray& aOther) { *this = aOther; }
+  nsStyleAutoArray& operator=(const nsStyleAutoArray& aOther) {
+    mFirstElement = aOther.mFirstElement;
+    mOtherElements = aOther.mOtherElements;
+    return *this;
+  }
+
+  bool operator==(const nsStyleAutoArray& aOther) const {
+    return Length() == aOther.Length() &&
+           mFirstElement == aOther.mFirstElement &&
+           mOtherElements == aOther.mOtherElements;
+  }
+  bool operator!=(const nsStyleAutoArray& aOther) const {
+    return !(*this == aOther);
+  }
+
+  size_t Length() const {
+    return mOtherElements.Length() + 1;
+  }
+  const T& operator[](size_t aIndex) const {
+    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
+  }
+  T& operator[](size_t aIndex) {
+    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
+  }
+
+  void EnsureLengthAtLeast(size_t aMinLen) {
+    if (aMinLen > 0) {
+      mOtherElements.EnsureLengthAtLeast(aMinLen - 1);
+    }
+  }
+
+  void SetLengthNonZero(size_t aNewLen) {
+    MOZ_ASSERT(aNewLen > 0);
+    mOtherElements.SetLength(aNewLen - 1);
+  }
+
+  void TruncateLengthNonZero(size_t aNewLen) {
+    MOZ_ASSERT(aNewLen > 0);
+    MOZ_ASSERT(aNewLen <= Length());
+    mOtherElements.TruncateLength(aNewLen - 1);
+  }
+
+private:
+  T mFirstElement;
+  nsTArray<T> mOtherElements;
+};
+
 struct nsStyleImageLayers {
   nsStyleImageLayers();
   nsStyleImageLayers(const nsStyleImageLayers &aSource);